The most important announcement at Microsoft's Windows 10 demo in Redmond the other day wasn't Windows 10 or Cortana, or even holograms. It was these four words: Windows as a Service.

Windows as a Service has been on the cards for some time. The idea is that Windows will transition away from being a monolithic product that sees periodic major releases to a product that's continually being updated and tweaked in the background.

Think Office 365 or Google's Chrome or Gmail.

Windows 10 will be the platform that paves the way for this change, but it's just the beginning, and transitioning Windows from monolithic releases to a service is going to mean big changes within the industry as a whole. And as always there are going to be pros and cons.

Pros

1: Microsoft is thinking about device lifespans rather than Windows versions

Microsoft's announcement that everyone running Windows 7, Windows 8.1, and Windows Phone 8.1 would be given a free upgrade to Windows 10 (for the first year following its release) was just the beginning. Terry Myerson, Microsoft's Executive Vice President of Operating Systems, went on to clarify:

"This is more than a one-time upgrade: once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device - at no cost. With Windows 10, the experience will evolve and get even better over time. We'll deliver new features when they're ready, not waiting for the next major release. We think of Windows as a Service - in fact, one could reasonably think of Windows in the next couple of years as one of the largest Internet services on the planet."

This represents a monumental change in how Microsoft looks at the hardware landscape.

It means that users always get access to the latest operating system, assuming that their hardware can handle it, and should help prevent a situation where people are running ancient versions of Windows. It's a model similar to how iOS, OS X, and in an ideal world, Android, works, and it is what consumers now expect.

People love free stuff, and the chance to get Windows 10 for free will no doubt generate buzz and goodwill. On top of that, the announcement that everyone gets a free upgrade means that PC sales won't get hit as people wait for the new release.

Cons

1: What about the future costs?

While Windows 10 will be offered free of charge to Windows 7, Windows 8.1, and Windows Phone 8.1 for a year, questions remain about the long-term model. Will this be a pay-per-device model, a yearly contract, or some hybrid depending on what the user wants?

Right now Microsoft is not willing to talk dollars and cents.

2: Will Windows be in a constant state of flux?

Another thing that worries me about Windows as a Service is that it means that the Windows platform will be in a constant state of flux. Buying a Windows license used to mean buying into a product lifecycle that was road-mapped out in advance.

Windows as a Service paves the way for new features to be shoehorned in as and when needed, and while some will no doubt enjoy that, it could be a nightmare for enterprise users. I can only hope that Microsoft offers a way for users to get critical system updates separate to non-critical updates. Time will tell.

3: OEMs are left out in the cold

Windows as a Service means no more big monolithic upgrade cycles, which in turn means that hardware OEMs don't get to enjoy the harvest time that follows. PC sales are already subdued and new releases of Windows don't seem to do that much to buoy them anyway, so the effect shouldn't be catastrophic for OEMs, but it could still mean trouble ahead, especially for the smaller players.

Microsoft is hoping that offering free Windows 10 upgrades people won't hold off buying new PCs in the run up to the release. However, canny buyers know that operating system updates can be hell, especially when you are dealing with OEM-built systems.

Being offered a free upgrade doesn't mean that the future won't be paved with driver issues and software incompatibilities.

5: Free stuff is still free

No matter how good Windows 10 is, giving it away for free is an indication of how much the landscape has changed over the past couple of decades. The days of people buying new PCs or overpriced upgrades are over. With so much shiny stuff out there for people to spend their money on, people are no longer willing to drop $100 on a new OS.

The unknowns

1: Microsoft's bottom line

No idea what this change in the way Windows is both developed and released will have on revenues. But traditionally Microsoft has enjoyed increased revenues around the time that new versions of Windows.

If Windows as a Service ends up being a one-time fee when the device is purchased, then revenues are at the mercy of sales. However, if it becomes a pay-to-play scenario where everyone 'rents' Windows for a fixed period then this could help spread revenue and smooth out the peaks and troughs.

2: Uncertainty

Right now the idea of a free upgrade to Windows 10 seems compelling enough, but beyond that there will be uncertainty. Enterprise users will be especially sensitive to any licensing changes that will result in increased costs.

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